1. Field of the Invention
The present invention relates to a large capacity bubble domain memory device comprising a large number of memory planes each thereof carrying a thin magnetic film for bubble domain and, on top of it, deposited thin ferromagnetic films of T-BAR, Y-BAR, etc. patterns, and more particularly to a rotating field driving apparatus for applying rotating fields in the plane of the thin magnetic film for bubble domain so as to transfer the bubble domains over the pattern formed on the memory plane by the thin ferromagnetic film.
2. Description of the Prior Art
Bubble domain memory devices which have been hitherto proposed generally comprise a memory element of a substantially cubical shape consisting of a plurality of planes each thereof carrying deposited thin ferromagnetic films on top of a thin magnetic film for bubble domain and coils wound on the memory element to apply a field in each of the three directions of the respective planes, i.e., the transversal or X direction, the longitudinal or Y direction and the vertical or Z direction. The memory planes used in such prior art devices consist, for example, of deposited T-BAR permalloy films placed on a large bubble domain magnetic film of about 2.5 centimeter square. Another construction of the memory plane consists of deposited T-BAR permalloy films placed on a large number of small bubble domain thin magnetic film pieces of 2 to 10 mm.sup.2 and these thin magnetic film pieces are mounted on an insulating substrate such as a ceramic substrate of about 5 cm.sup.2. The field applied in the Z direction is a biasing field of a predetermined magnitude which is employed to produce bubble domains in the bubble domain thin magnetic film pieces. This biasing field may be applied by a permanent magnet. The X and Y fields are rotating in-plane fields which vary sinusoidally with a 90.degree. phase difference therebetween or their modified forms which vary in a pulse-like or sawtoothwave-like manner, and the fields rotate through 360.degree..
While large capacity bubble domain memory devices of the type constructed as described have been proposed, no driving methods have been disclosed for such memory devices due to the elements being of recent discovery. It is known that the energy of a magnetostatic field is proportional to its cubic volume and the impedance of a coil is proportional to a driving frequency. Thus, there is naturally an upper limit to the magnitude of a frequency at which a memory element of great volume can be driven.
If it is assumed that a uniform field H is applied in a cubical space with the sides a and the flux is totally produced by a coil having N turns, then the voltage E applied to the coil is given by the following equation (1): ##EQU1## where .phi. is the total flux and B is the flux density. If it is also assumed that the magnetomotive force NI of the coil is whooly acting on the cube, then we obtain: EQU NI = H .sup.. a 2 EQU B = .mu..sub.o H 3
then from the equations (1) to (3), we obtain: ##EQU2## Therefore ##EQU3## Now assume that the following relations hold: EQU H = H.sub.R sin .omega.t EQU I = I.sub.R sin .omega.t 6 EQU E = E.sub.R cos .omega.t
Then the equation (5) is rewritten as: EQU E.sub.R .sup.. I.sub.R = .mu..sub.o .sup.. a.sup.3 . .omega.H.sub.R.sup.2 7
therefore, as the sum of the coil current I.sub.R in the memory element and the peak value of voltage E.sub.R is proportional to the volume a.sup.3, frequency f (= .omega./2.pi.) and the square of field H.sub.R.sup.2, it is evident that an upper limit is naturally set on the value of frequency at which a bulky memory element can be driven.